"Side by side" could be understand differently; there are many subtle detail, such as vertical alignment; do you need to fill in the horizontal space or not, how; if you need a match of height or not, and so on. Thy this pretty elegant ans simple solution.
<html>
<head>
<title>Some title</title>
<style type="text/css">
div { padding: 1em; }
div.parent { display: table; }
div.left { display: table-cell; border: solid 2px red; }
div.right { display: table-cell; border: solid 2px green; }
</style>
</head>
<body>
<div class="parent">
<div class="left">Left<br/><br/>…to demonstrate<br/>different<br/>contect height<br/></div>
<div class="right">Right</div>
</div>
</body>
</html>
If your requirements are different, feel free to ask further questions.
—SA